Method of fabricating multilayer ceramic substrate

ABSTRACT

A method of fabricating a multilayer ceramic substrate includes stacking one or a plurality of unfired ceramic greensheets on one or both sides of a previously fired ceramic substrate, thereby forming a stack, each unfired ceramic greensheet having a firing temperature substantially equal to or lower than a firing temperature of the previously fired ceramic substrate, stacking a restricting greensheet on the unfired ceramic greensheet composing an outermost layer of the stack, the restricting greensheet having a higher firing temperature than each unfired ceramic greensheet, firing the stack at the firing temperature of the unfired ceramic green sheets with or without pressure applied via the restricting greensheet while the stack is under restriction by the restricting greensheet, thereby integrating the stack, and eliminating remainders of the restricting greensheet after the firing step.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method of fabricating a multilayerceramic substrate by stacking unfired ceramic greensheets on either oneor both of sides of a previously fired ceramic substrate.

[0003] 2. Description of the Related Art

[0004] Multilayer ceramic substrates have conventionally been fabricatedby stacking greensheets. In the greensheet stacking process, after viaholes have been formed in a plurality of ceramic greensheets, the viaholes of each sheet are filled with conductor paste so that viaconductors are formed, and a wiring pattern is printed on each ceramicgreensheet using conductor paste. Thereafter, these ceramic greensheetsare made by the greensheet stacking process and thermo-compressionbonding into a raw substrate. Subsequently, the raw substrate is firedto be fabricated into a multilayer ceramic substrate.

[0005] However, about 15 to 30% firing shrinkage occurs in a process offiring the raw substrate. This renders control of dimensional accuracyin the substrate difficult. Moreover, since a shrinkage stress of bothsides of the substrate become non-uniform in a multilayer ceramicsubstrate which has irregularity such as cavity, warp tends to occur inthe fired substrate. In particular, warp becomes larger in a bottom ofthe cavity.

[0006] Further, firing temperatures of both types of ceramic greensheetsneed to be equalized when a composite multilayer ceramic substrate isfabricated by stacking insulating ceramic greensheets and other ceramicgreensheets made from material differing in a dielectric substance andmagnetic substance. Furthermore, since delamination needs to beprevented by reducing difference in the behavior in the firingshrinkage, the freedom in material selection and accordingly the freedomin the design of the substrate are very limited.

[0007] A firing process has recently been proposed reducing the firingshrinkage of the substrate thereby improving the dimensional accuracy ofthe substrate, as shown in JP-A-2001-267743. In this firing process, anunfired ceramic greensheet on which a wiring pattern is previouslyprinted is stacked on a previously fired alumina substrate to be furtherprocessed by thermo-compression bonding. A stack of the greensheet andthe substrate is then fired to be fabricated into a multilayer ceramicsubstrate. In this method, the firing shrinkage of each ceramicgreensheet is restrained by the previously fired alumina substrate,whereby the firing shrinkage of the entire substrate is reduced.

[0008] However, the results of an experiment conducted by the inventorreveals that a shrinking force of the greensheet is so large that thefiring shrinkage thereof cannot sufficiently be restrained even whenonly the previously fired alumina substrate is applied to one side ofthe ceramic greensheet. As a result, peeling occurs between a firedlayer of ceramic greensheet and previously fired alumina substrate, acrack occurs on the fired layer of ceramic greensheet, and warp occursin the substrate, whereupon a yield of the products is reduced.

[0009] Further, as a firing process reducing the firing shrinkage of thesubstrate thereby to improve the dimensional accuracy thereof, processesfor firing under pressure have been developed as shown in WO91/10630 andJP-A-9-92983. In these firing processes, a restricting aluminagreensheet which is not fired at a firing temperature (800 to 1000° C.)of a low-temperature firable ceramic is stacked on both sides of alow-temperature firable ceramic substrate (raw substrate) before thelow-temperature firable ceramic is fired. In this state, the rawsubstrate is fired at a temperature ranging from 800 to 1000° C. underpressure. Subsequently, remainders of the restricting aluminagreensheets are eliminated from the sides of the fired substrate by ablasting process etc., whereby a low-temperature firable ceramicsubstrate is fabricated.

[0010] However, when a low-temperature firable ceramic substrate with acavity is fired by the above-mentioned firing under pressure, pressureapplied via the restricting alumina greensheet to a cavity area actsconcentrically on a peripheral edge of the cavity, and no pressure isapplied to a bottom of the cavity. As a result, the cavity bottom iswarped into a convexity and accordingly, the dimensional accuracy of thecavity cannot be ensured.

SUMMARY OF THE INVENTION

[0011] Therefore, a primary object of the present invention is toprovide a method of fabricating multilayer ceramic substrate which canimprove, to a large extent, the freedom in the selection of materialwith respect to a firing temperature, firing shrinkage characteristic,etc. of a ceramic material forming each layer of the multilayer ceramicsubstrate, and which can fabricate a multilayer ceramic substrate with ahigh dimensional accuracy without delamination and warp, which substrateis difficult to fabricate in the conventional fabricating methods.

[0012] Another object of the invention is to provide a method offabricating a multilayer ceramic substrate which can prevent the cavitybottom from being warped into the convexity and which can fabricate amultilayer ceramic substrate with a high dimensional accuracy.

[0013] To achieve the primary object, the present invention provides amethod of fabricating a multilayer ceramic substrate comprising stackingone or a plurality of unfired ceramic greensheets on one or both sidesof a previously fired ceramic substrate, thereby forming a stack, eachunfired ceramic greensheet having a firing temperature substantiallyequal to or lower than a firing temperature of the previously firedceramic substrate, stacking a restricting greensheet on the unfiredceramic greensheet composing an outermost layer of the stack, therestricting greensheet having a higher firing temperature than eachunfired ceramic greensheet, firing the stack at the firing temperatureof the unfired ceramic green sheets with or without pressure applied viathe restricting greensheet while the stack is under restriction by therestricting greensheet, thereby integrating the stack, and eliminatingremainders of the restricting greensheet after the firing step.

[0014] The present invention is characterized in that the unfiredceramic greensheets are stacked on the previously fired substrate sothat the stack is formed and that the restricting greensheet is stackedon the stack and the stack is fired with or without pressure beingapplied while being under restriction by the restricting greensheet.Consequently, firing shrinkage, warp or other deformation of the unfiredceramic greensheet in the X and Y directions is restrained substantiallyuniformly at both sides thereof by the restricting greensheet and thepreviously fired substrate respectively during the firing step. As aresult, a multilayer ceramic substrate with a good dimensional accuracyand no delamination nor warp can be fabricated. Further, this method isfree from differences in the firing temperature, firing shrinkagecharacteristic, etc. between the previously fired substrate and theunfired ceramic greensheet. Consequently, the degree of freedom can beimproved in the selection of material in view of a firing temperature,firing shrinkage characteristic, etc. of a ceramic material composingeach layer of the multilayer ceramic substrate. Accordingly, amultilayer ceramic substrate with good dimensional accuracy but withoutdelamination or warp can be fabricated although it has been difficult tofabricate such a multilayer ceramic substrate in the conventionalfabricating method.

[0015] In the step of stacking the unfired ceramic greensheet on thepreviously fired substrate, the unfired ceramic greensheet may merely belaid on the previously fired substrate. The reason for this is that theceramic greensheets and previously fired substrate can be bondedtogether under heat and pressure when the stack is heated under pressureat a subsequent step. Generally, however, it is preferable that thepreviously fired substrate and the unfired ceramic greensheets aretemporarily tacked together, for example, by means of thermo-compressionbonding or adhesive agent, in the step where the unfired ceramicgreensheet is stacked on the previously fired substrate. As a result,since misregistration is prevented between the previously firedsubstrate and the unfired ceramic greensheet, the stack can be treatedeasily at a subsequent step. Further, since a bond strength is improvedbetween the ceramic greensheet and the previously fired substrate, theycan be prevented from delamination and warp.

[0016] Further, the restricting greensheet may merely be laid on theunfired ceramic greensheet at a step where the restricting greensheet isstacked on the unfired ceramic greensheet composing the outermost layerof the stack. The reason for this is that the restricting greensheet andthe outermost unfired ceramic greensheet can be bonded together underheat and pressure when heated under pressure at a subsequent step.Generally, however, it is preferable that the restricting greensheet andthe unfired ceramic greensheet are temporarily tacked together in thestep where the restricting greensheet is stacked on the unfired ceramicgreensheets. In this case, a restricting force of the restrictinggreensheet can be applied to the unfired ceramic greensheet andaccordingly, a multilayer ceramic substrate with good dimensionalaccuracy but without delamination or warp can be fabricated although ithas been difficult to fabricate such a multilayer ceramic substrate inthe conventional fabricating methods.

[0017] When the previously fired substrate and the unfired ceramicgreensheet are made from a same ceramic material, the multilayer ceramicsubstrate with a single ceramic composition can be fabricated in whichelectrical characteristics such as an insulating characteristic aresubstantially uniform over the all layers. Alternatively, the previouslyfired substrate and the unfired ceramic greensheet may be made fromdifferent ceramic materials from each other, and the ceramic materialsmay be selected so that the firing temperature of the unfired ceramicgreensheet is equal to or lower than a firing temperature of thepreviously fired substrate. Consequently, a composite multilayer ceramicsubstrate containing various functional materials can be fabricatedalthough it has been difficult to fabricate such a multilayer ceramicsubstrate in the conventional fabricating methods.

[0018] The unfired ceramic greensheet is preferably made from alow-temperature fired ceramic material which is fired at a temperatureequal to or lower than 1000° C. Consequently, ceramics which is lessexpensive and has a high mechanical strength, for example, aluminagreensheets, can be used as the restricting greensheet, and materialswith respective low melting points, for example, Ag, Au and Cu systems,can be used as a wiring conductor to be printed on the unfired ceramicgreensheets. Each of these materials has good electrical characteristicssuch as a low resistance value.

[0019] The previously fired substrate and/or the unfired ceramicgreensheet are preferably made from a ceramic having any one ofinsulating, dielectric, magnetic, piezoelectric and resistive functions.In this case, the insulating ceramic refers to a ceramic used to form aninsulating layer of the substrate. For example, the insulating ceramicincludes low-temperature firable ceramics, high-temperature firableceramics such as alumina. Since the previously fired substrate is firedindependently, every type of ceramic can be used. Accordingly, theunfired ceramic greensheet can be formed from a ceramic material firedat a temperature equal to or lower than a firing temperature of thepreviously fired substrate.

[0020] To achieve the second object, the present invention provides amethod of fabricating a multilayer ceramic substrate having a cavity,wherein the previously fired substrate is placed on a bottom of aportion where the cavity is to be formed, and an opening for forming thecavity is formed in the unfired ceramic greensheet which is stacked onthe previously fired substrate, before or after the stacking. Whenformed in the unfired ceramic greensheet before the stacking, theopening for forming the cavity is formed by punching etc. simultaneouslywith the forming of via holes. Further, when formed in the unfiredceramic greensheet after the stacking, a ceramic green sheet containinga photosensitive resin is formed, and the opening for forming the cavityis formed on the ceramic greensheet by a technique for forming adesirable opening by photo-etching.

[0021] When the previously fired substrate is placed on the bottom ofthe cavity and the multilayer ceramic substrate with the cavity is firedwhile being restricted, the cavity bottom can be prevented from beingwarped into the convex shape and a dimensional accuracy of the cavitycan be ensured.

[0022] Further, the following describes a case where a multilayerceramic substrate having a stepped cavity is fabricated. That is, everytime a predetermined number of the unfired ceramic greensheetscorresponding to a number of layers of one step of the stepped cavity isstacked on the previously fired substrate, the restricting greensheetare stacked on the unfired ceramic greensheet into the stack, and thestack is fired while being under restriction by the restricting ceramicgreensheet, thereby fabricating a new fired substrate with one step ofcavity. Thereafter, another predetermined number of the unfired ceramicgreensheets corresponding to a number of layers of a subsequent step ofthe stepped cavity and the restricting greensheet are stacked on saidnew fired substrate into a stack, and the stack is fired while beingunder restriction by the restricting greensheet, after remainders of therestricting greensheet for fabrication of said new fired substrate hasbeen eliminated, repeatedly so that the multilayer ceramic substratehaving the stepped cavity is fabricated. Consequently, each steppedportion of the cavity has a good flatness and a good dimensionalaccuracy. In this case, too, the opening for forming the cavity may beformed in the unfired ceramic greensheet which is stacked on thepreviously fired substrate, before or after the stacking.

[0023] The conductor pattern may be printed on the unfired ceramicgreensheet after the stacking. However, a conductor pattern co-firablewith the unfired ceramic greensheet may be printed on said unfiredceramic greensheet and thereafter, said unfired ceramic greensheet maybe stacked on one or both sides of the previously fired ceramicsubstrate. Consequently, the printing and stacking can efficiently becarried out when a plurality of the unfired ceramic green sheets arestaked on the previously fired substrate.

[0024] The multilayer ceramic substrate preferably has a surface and aback each one of which is dissimilar to the other. Even if themultilayer ceramic substrate has such a complicated structure as tocause a warp or other deformation, warp or other deformation can beprevented and the dimensional accuracy thereof can be improved.

[0025] Further, a thick film resistor is preferably formed on the firedsubstrate and the thick film resistor is preferably trimmed foradjustment of a resistance value, and thereafter the unfired ceramicgreensheet is preferably stacked on the fired substrate. Consequently, amultilayer ceramic substrate with an integrated thick film resistorwhose resistance value is adjusted by trimming can be fabricated withgood dimensional accuracy.

[0026] The multilayer ceramic substrate fabricated in theabove-described method of the present invention can be used as thosewith various purposes. For example, the multilayer ceramic substrate maybe used to manufacture module semiconductor devices such ascommunication module products, or vehicle module products.

BRIEF DESCRIOPTION OF THE DRAWINGS

[0027] Other objects, features and advantages of the present inventionwill become clear upon reviewing of the following description of theembodiments, made with reference to the accompanying drawings, in which:

[0028]FIGS. 1A, 1B and 1C are views explaining the processing in themethod of fabricating a multilayer ceramic substrate having one sideformed with a cavity in accordance with a first embodiment of theinvention;

[0029]FIG. 2 is a flowchart explaining fabricating steps in the methodof the first embodiment;

[0030]FIG. 3 is a view explaining the processing in the method offabricating a multilayer ceramic substrate having one side formed with acavity whose depth corresponds to two layers;

[0031]FIG. 4 is a view explaining the processing in the method offabricating a multilayer ceramic substrate having both sides formed withrespective cavities in accordance with a second embodiment of theinvention;

[0032]FIGS. 5A, 5B, 5C and 5D are views explaining the processing in themethod of fabricating a multilayer ceramic substrate having a steppedcavity in accordance with a third embodiment of the invention;

[0033]FIG. 6 is a view explaining the processing in the method offabricating a composite multilayer ceramic substrate in accordance witha fourth embodiment of the invention; and

[0034]FIGS. 7A and 7B are views explaining the processing in the methodof fabricating a composite multilayer ceramic substrate having bothsides formed with respective cavities in accordance with a fifthembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0035] A first embodiment in which the invention is applied to a methodof fabricating a multilayer ceramic substrate having one side formedwith a cavity will be described with reference to FIGS. 1A to 3. Amultilayer ceramic substrate 11 having one side formed with a cavity isfabricated in the embodiment. In the substrate 11, one or a plurality ofunfired low-temperature firable ceramic greensheets 13 are stacked andlaminated on a previously fired substrate 12, so that a stack is formed,as shown in FIGS. 1C and 3. Two restricting greensheets 15 are furtherstacked on both sides of the laminate respectively. The stack is thenfired at a temperature ranging from 800 to 1000° C. while being underrestriction by the restricting greensheets 15 (with or without pressureapplied). The previously fired substrate 12 forms a bottom of the cavity14.

[0036] The multilayer ceramic substrate 11 is fabricated through thefollowing steps. Firstly, the previously fired substrate 12 is prepared.The substrate 12 is formed by firing a ceramic substrate and may be amonolayer or multilayer. A ceramic material formed into the substrate 12may be an insulating ceramic, dielectric ceramic, magnetic ceramic,piezoelectric ceramic or a ceramic with a resistor. What is essential isthat a ceramic material should be fired at a temperature equal to orhigher than a firing temperature of the low-temperature firable ceramicgreensheet 13. Further, when the previously fired substrate 12 is amultilayer substrate, each layer may be formed from the same ceramic.The layers may include those formed from different ceramics which areco-firable.

[0037] The insulating ceramic is used for forming an insulating layer ofthe substrate and includes a low-temperature firable ceramic or ahigh-temperature firable ceramic such as alumina. A low-temperaturefirable ceramic, when formed into the previously fired substrate 12, maybe of the same type as the low-temperature firable ceramic greensheet13. Moreover, another type of low-temperature firable ceramic fired at atemperature equal to higher than a firing temperature of thelow-temperature firable ceramic greensheet 13. Further, a thick-filmconductor or thick-film resistor of the RuO₂ system etc. may be formedon a surface of the previously fired substrate 12 by simultaneous firingor subsequent processing. When a thick-film resistor is formed on thesurface of the previously fired substrate 12, the thick-film resistormay be trimmed for adjustment of a resistance value thereof before thelow-temperature firable ceramic greensheet 13 is stacked on thepreviously fired substrate 12.

[0038] Next, the low-temperature firable ceramic greensheet 13 isprepared. A low-temperature firable ceramic material for the greensheet13 may be a blend of 50 to 65 weight % (preferably 60 weight %) of glassof CaO—SiO₂—Al₂O₃—B₂O₃ system and 50 to 35 weight % (preferably 40weight %) of alumina, for example. Other low-temperature firable ceramicmaterials include a blend of glass of MgO—SiO₂—Al₂O₃—B₂O₃ system andalumina, a blend of glass of PbO—SiO₂—B₂O₃ system and alumina, or alow-temperature firable ceramic material which can be fired at atemperature ranging from 800 to 1000° C., for example, a crystallizedglass of cordierite system.

[0039] In making the low-temperature firable ceramic greensheet 13, abinder (for example, polyvinyl butyral, acrylic resin, etc.), a solvent(for example, toluene, xylene, butanol, etc.) and a plasticizer areblended with the aforementioned low-temperature firable ceramicmaterial. These materials are sufficiently agitated and blended to bemade into slurry. The slurry is further made into a tape of thelow-temperature firable ceramic greensheet 13 by a doctor blade process.The tape of the low-temperature firable ceramic greensheet 13 is cutinto a predetermined size and subsequently, an opening 14 a to be formedinto a cavity and via holes (not shown) are formed by punching atrespective predetermined positions in each green sheet 13.

[0040] Thereafter, the process advances to a printing step where the viaholes of the greensheet 13 are filled with a conductor paste of metalwith a low melting point such as Ag, Ag/Pd, Au, Ag/Pt, Cu, etc. Further,when a plurality of low-temperature firable greensheets 13 are stackedon the previously fired substrate 12 as shown in FIG. 3, an inner layerconductor pattern (not shown) is printed on each low-temperature firablegreensheet 13 stacked on an inner layer by the screen printing processusing a conductor paste of metal with a low melting point such as Ag,Ag/Pd, Au, Ag/Pt, Cu, etc. A surface layer conductor pattern (not shown)is printed on the low-temperature firable greensheet 13 constituting asurface or uppermost layer by the screen printing process using the sametype of the conductor paste of metal with a low melting point. Thesurface layer conductor pattern may be printed after a stack is firedunder while being restricted. The above-described printing process iseliminated when a multilayer ceramic substrate to be fabricated has novia holes or inner layer conductor pattern.

[0041] Subsequently to the printing step, the process advances to astacking step where one or a plurality of low-temperature firablegreensheets 13 are stacked on the previously fired substrate 12. Aresultant laminate is temporarily tacked by thermo-compression bonding.The conditions of the thermo-compression bonding preferably include anapplied pressure ranging from 10⁵ to 10⁷ Pa and a heating temperatureranging from 60 to 150° C. The low-temperature firable greensheets 13may merely be stacked on the previously fired substrate 12 in thestacking step and accordingly, the thermo-compression bonding may beeliminated.

[0042] Restricting greensheets 15 are then stacked on both sides of thestack respectively to be temporarily tacked by the thermo-compressionbonding under the above-mentioned conditions. Even when thethermo-compression bonding has not been carried out in the stackingstep, the low-temperature firable ceramic greensheets 13 and thepreviously fired substrate 12 can be bonded by the thermo-compressionbonding in the process of thermo-compression bonding for the restrictinggreen sheets 15. The thermo-compression bonding for the restrictinggreensheets 15 may be eliminated when firing is carried out underpressure.

[0043] When the low-temperature firable ceramic greensheet 13 is stackedonly on one side of the previously fired substrate 12, the restrictinggreensheet 15 is stacked only on one side on which the greensheet 13 isstacked, and the restricting greensheet 15 to be stacked on the otherside may be eliminated, as shown in FIGS. 1C and 3. The reason for thisis that the previously fired substrate 12 serves to restrain the firingshrinkage of the greensheets 13 during the firing of the stack underrestriction as each greensheet 15 does. In this case, a high-temperaturefirable ceramic such as alumina, zirconia, magnesia, etc. is used forthe restricting greensheet 15. The high-temperature firable ceramic isnot fired at the firing temperature of the low-temperature firableceramic ranging from 800 to 1000° C. A binder (for example, resin ofpolyvinyl butyral, acrylic or nitrocellulose system, etc.), a solvent(for example, toluene, xylene, butanol, etc.) and a plasticizer areblended with a powder of the high-temperature firable ceramic. Thesematerials are sufficiently agitated and blended to be made into slurry.The slurry is further made into a tape of the restricting greensheet 15by the doctor blade process.

[0044] Subsequently, the stack on which the restricting greensheets 15are stacked is interposed between two porous setter plates (not shown)made from alumina, SiC, etc. The stack is fired at a firing temperatureof the low-temperature firable ceramic greensheet 13 ranging from 800 to1000° C. while being under pressure of 10⁵ to 10⁷ Pa. Alternatively, thestack may be fired without being pressurized. In this case, therestricting greensheets 15 need to be bonded to the low-temperaturefirable ceramic greensheet 13 by the thermo-compression bonding duringthe stacking of the restricting greensheets 15. The restrictinggreensheet 15 (high-temperature firable ceramic such as alumina) isfired at a temperature ranging from 1300 to 1600° C. Accordingly, therestricting greensheet 15 remains unfired when fired at a temperatureranging from 800 to 1000° C. Organic substances such as the bindercontained in the restricting greensheet 15 are thermally decomposed inthe firing process to scatter away, thereby remaining as ceramic powder.

[0045] After the firing, the remainder (ceramic powder) of therestricting greensheet 15 adherent to both sides of the fired substrate11 is eliminated by blasting or buffing, whereupon the multilayerceramic substrate 11 with a cavity in one side thereof is completed.

[0046] According to the foregoing embodiment, the unfiredlow-temperature firable ceramic greensheet 13 is stacked on thepreviously fired substrate 12 so that the stack is fabricated. Therestricting greensheets 15 are stacked on the stack, which is then firedunder pressure or no pressure while being restricted by the restrictinggreensheets 15. Accordingly, firing shrinkage, warp or other deformationof the unfired low-temperature firable ceramic greensheet 13 isrestrained substantially uniformly at both sides thereof by therestricting greensheets 15 and the previously fired substrate 12 duringthe firing step. Consequently, the multilayer ceramic substrate 11without delamination or warp can be fabricated.

[0047] Moreover, when the multilayer ceramic substrate 11 with thecavity 14 is fabricated, the previously fired substrate 12 is located onthe bottom of the cavity 14. When the stack is then fired while beingrestricted, the cavity bottom can be prevented from being warped into aconvexity. Moreover, the dimensional accuracy of the cavity 14 can beensured by the firing of the stack under restriction. Consequently, themultilayer ceramic substrate 11 with a high quality of cavity 14 can befabricated. Accordingly, even when a bare semiconductor chip is mountedon the bottom of the cavity 14 in a flip-chip manner, bonding orconnection between the bare chip and a conductive pad of the cavitybottom can accurately be carried out since the cavity bottom has nowarp. Consequently, the reliability of the flip-chip mounting can beimproved.

[0048] Further, the above-described fabricating method is free fromdifferences in the firing temperature, firing shrinkage characteristic,etc. between the previously fired substrate 12 and the unfired ceramicgreensheet 13. Consequently, a degree of freedom can be improved in theselection of material in view of a firing temperature, firing shrinkagecharacteristic, etc. of a ceramic material composing each layer of themultilayer ceramic substrate 11. Accordingly, a multilayer ceramicsubstrate 11 with good dimensional accuracy but without delamination orwarp can be fabricated although it has been difficult to fabricate sucha multilayer ceramic substrate in the conventional fabricating method.

[0049] The cavity opening 14 a is formed in the low-temperature firableceramic greensheet 13 by means of punching before the stacking andlaminating in the foregoing embodiment. However, the cavity opening 14 amay be formed in the greensheet 13 after the stacking using aphotolithographic technique, instead.

[0050]FIG. 4 illustrates a second embodiment in accordance with theinvention. The low-temperature firable ceramic greensheet 13 is stackedonly on one side of the previously fired substrate 12 in the firstembodiment. However, one or a plurality of low-temperature firableceramic greensheets 13 are stacked on each of both sides of thepreviously fired substrate 12 as shown in FIG. 4. In this case, theunfired low-temperature firable ceramic greensheets 13 are stacked onboth sides of the previously fired substrate 12 so that a stack isformed and thereafter, the restricting greensheets 15 are stacked onboth sides of the stack respectively. The fabricating method of thesecond embodiment is the same as that of the foregoing embodiment inother respects.

[0051] Even when the multilayer ceramic substrate 11 with cavitiesformed in both sides thereof is fabricated as in the second embodiment,the previously fired substrate 12 is positioned on the bottom of thecavity 14 and the stack is then fired under restriction. Consequently,the cavity bottom can be prevented from being warped into a convexity,the multilayer ceramic substrate 11 with both cavities having gooddimensional accuracy can be fabricated.

[0052]FIGS. 5A to 5D illustrate a third embodiment of the invention. Thecavity 14 is formed in the multilayer ceramic substrate 11 in each ofthe first and second embodiments. The cavity 14 has a stepless simpleconfiguration in each embodiment. In this case, no problem ofdeformation of the cavity 14 arises even when a plurality of unfiredlow-temperature firable ceramic greensheets 13 are simultaneouslystacked on the previously fired substrate 12 to be bonded together bythe thermo-compression bonding as shown in FIG. 3.

[0053] On the other hand, a plurality of low-temperature firable ceramicgreensheets 13 having respective cavity-forming openings 16 a withdifferent sizes are used when a stepped cavity 16 is formed in amultilayer ceramic substrate. In this case, when the low-temperaturefirable ceramic greensheets 13 are simultaneously stacked to be bondedtogether by the thermo-compression bonding as shown in FIG. 5D, there isa possibility that a resultant pressure may deform the stepped portionsof the stepped cavity 16. The third embodiment is directed to a solutionof the above-described problem. When a multilayer ceramic substrate 17having the stepped cavity 16 is fabricated, an unfired low-temperaturefirable ceramic greensheet 13 the number of which corresponds to that oflayers forming one step of the cavity 16 is stacked on the previouslyfired substrate 12 as shown in FIG. 5A. Subsequently, the restrictinggreensheet 15 is stacked on the unfired low-temperature firable ceramicgreensheet 13 to be fired under restriction, whereby a new previouslyfired substrate 17 a having a cavity for one step is fabricated. Theremainder of the restricting greensheet 15 is then eliminated by theblasting process and thereafter, a subsequent unfired low-temperaturefirable ceramic greensheet 13 the number of which corresponds to that oflayers forming one step of the cavity 16 is stacked on the new substrate17 a to be fired under restriction, as shown in FIG. 5B. Theabove-described operation is repeated at the number of timescorresponding to the number of remaining steps, so that the multilayerceramic substrate 17 having the stepped cavity 16 is fabricated.

[0054] When the multilayer ceramic substrate 17 having the steppedcavity 16 is fabricated in the foregoing method, the configuration ofeach step can be prevented from deformation and a good dimensionalaccuracy can be achieved. In this case, too, the cavity-forming opening16 a may be formed in the unfired low-temperature ceramic greensheet 13either before or after stacking.

[0055]FIG. 6 illustrates a fourth embodiment of the invention. In thefourth embodiment, a previously fired substrate 18 is made from afunctional material other than the insulating ceramic, for example,dielectric ceramic, magnetic ceramic, piezoelectric ceramic or resistiveceramic. Although a plurality of previously fired substrates 18 are usedin the fourth embodiment, a single substrate 18 may be used, instead.When a plurality of substrates 18 are used, one or a plurality ofunfired ceramic greensheets 13 need to be interposed between eachsubstrate 18 and the adjacent one. Further, the previously firedsubstrate 18 and the unfired ceramic greensheet 13 are stacked to bebonded by the thermo compression bonding, and subsequently, therestricting greensheets 15 are stacked on both sides of the stack to befired under restriction, whereby a composite multilayer ceramicsubstrate 19 is fabricated.

[0056] The foregoing method is free from differences in the firingtemperature, firing shrinkage characteristic, etc. between thepreviously fired substrate 18 and the unfired ceramic greensheet 13.Consequently, the degree of freedom can be improved in the selection ofmaterial in view of a firing temperature, firing shrinkagecharacteristic, etc. of a ceramic material composing each layer of thecomposite multilayer ceramic substrate 19. Accordingly, the compositemultilayer ceramic substrate 19 with various integrated functionalmaterials can be fabricated with good dimensional accuracy but withoutdelamination or warp although it has been difficult to fabricate such acomposite multilayer ceramic substrate in the conventional fabricatingmethod.

[0057]FIGS. 7A and 7B illustrate a fifth embodiment of the invention.The fifth embodiment is directed to a case where a composite multilayerceramic substrate 20 with a cavity 14 is fabricated. In this case, thecomposite multilayer ceramic substrate fabricated in the same method asthe method of the fourth embodiment is used as the previously firedsubstrate 19. One or a plurality of low-temperature firable ceramicgreensheets 13 are stacked on both sides of the previously firedsubstrate 19. The cavity-forming opening 14 a is formed in eachlow-temperature firable ceramic greensheet 13 before or after stacking.The low-temperature firable ceramic greensheets 13 are stacked on bothsides of the previously fired substrate 19 respectively to be bondedtogether by the thermo-compression bonding. Subsequently, therestricting greensheets 15 are stacked on both sides of the stack to befired under restriction, whereby the composite multilayer ceramicsubstrate 20 with the cavity 14 is fabricated.

[0058] The structure of the multilayer ceramic substrate to which themethod of the present invention is applied should not be limited tothose described in the foregoing embodiments. Changes can be maderegarding the number of previously fired substrates, a location where agreensheet is stacked, the number of low-temperature firable ceramicgreensheets, a configuration of the cavity and the type of ceramicmaterial composing each layer of the previously fired substrate, etc.

[0059] The multilayer ceramic substrates fabricated in the respectivemethods of the foregoing embodiments can be used as multilayer circuitboards for various purposes. For example, semiconductor devices such ascommunication module products and vehicle module products may beproduced using the multilayer ceramic substrate.

[0060] The inventor measured an amount of warp on the bottom of thecavity regarding the multilayer ceramic substrate fabricated undervarious conditions in the fabricating method of each embodiment. TABLES1 and 2 show the results of measurement. TABLE 1 Embodiments PreviouslyUnfired fired substrate ceramic greensheet Restricting Sample ThicknessThickness greensheet No. Material after firing Material before firingMaterial 1 (1) 0.4 mm (1) 0.5 mm Alumina 2 ↑ 0.4 mm ↑ 0.5 mm Zirconia3 ↑ 0.4 mm ↑ 0.5 mm Magnesia 4 ↑ 0.4 mm ↑ 0.2 mm Alumina 5 ↑ 0.4 mm ↑0.5 mm Alumina 6 ↑ 0.4 mm ↑ 0.7 mm Alumina 7 ↑ 0.2 mm ↑ 0.5 mm Alumina 8↑ 0.4 mm ↑ 0.5 mm Alumina 9 ↑ 0.6 mm ↑ 0.5 mm Alumina 10  ↑ 0.4 mm ↑ 0.5mm Alumina 11  ↑ 0.4 mm ↑ 0.5 mm Alumina 12  ↑ 0.4 mm ↑ 0.5 mm Alumina13  ↑ 0.4 mm ↑ 0.5 mm Alumina 14  ↑ 0.4 mm ↑ 0.5 mm Alumina Conditionsfor thermo Sam- compression bonding Pressure Cavity ple Temper- appliedAmount of No. ature Pressure during firing Structure warp in bottom 160° C. 10⁶ Pa 10⁵ Pa one side ≦5 μm 2 60° C. 10⁶ Pa 10⁶ Pa one side ≦5μm 3 60° C. 10⁶ Pa 10⁷ Pa one side ≦5 μm 4 60° C. 10⁶ Pa 10⁵ Pa one side≦5 μm 5 60° C. 10⁶ Pa 10⁶ Pa one side ≦5 μm 6 60° C. 10⁶ Pa 10⁷ Pa oneside ≦5 μm 7 60° C. 10⁶ Pa 10⁵ Pa one side ≦5 μm 8 60° C. 10⁶ Pa 10⁶ Paone side ≦5 μm 9 60° C. 10⁶ Pa 10⁷ Pa one side ≦5 μm 10  60° C. 10⁶ Pa 0one side ≦5 μm 11  60° C. 10⁶ Pa 10³ Pa one side ≦5 μm 12  60° C. 10⁶ Pa10⁶ Pa one side ≦5 μm 13  60° C. 10⁶ Pa 10⁶ Pa one side ≦5 μm 14  100°C.  10⁶ Pa 10⁶ Pa one side ≦5 μm

[0061] TABLE 2 Embodiments Previously Unfired fired substrate ceramicgreensheet Restricting Sample Thickness Thickness greensheet No.Material after firing Material before firing Material 15 (1) 0.4 mm(1) 0.5 mm Alumina 16 ↑ 0.4 mm ↑ 0.5 mm Alumina 17 ↑ 0.4 mm ↑ 0.2 mmAlumina 18 ↑ 0.4 mm ↑ 0.5 mm Alumina 19 ↑ 0.4 mm ↑ 0.7 mm Alumina 20Alumina 0.3 mm ↑ 0.5 mm Alumina 21 Ferrite 0.3 mm ↑ 0.5 mm Aluminasystem 22 Relaxor 0.3 mm ↑ 0.5 mm Alumina system 23 (2) 0.3 mm (3) 0.5mm Alumina 24 Barium titanate 0.3 mm (1) 0.5 mm Alumina system 25Relaxor 0.3 mm ↑ 0.5 mm Alumina system 26 Relaxor 0.3 mm ↑ 0.5 mmAlumina system 27 (1) 0.4 mm (3) 0.5 mm Zirconia 28 (3) 0.4 mm ↑ 0.5mm Zirconia Conditions for thermo Sam- compression bonding PressureCavity ple Temper- applied Amount of No. ature Pressure during firingStructure warp in bottom 15 60° C. 10⁶ Pa 10⁶ Pa one side ≦5 μm 16 60°C. 10⁷ Pa 10⁶ Pa one side ≦5 μm 17 60° C. 10⁶ Pa 10⁵ Pa Both sides ≦5 μm18 60° C. 10⁶ Pa 10⁶ Pa Both sides ≦5 μm 19 60° C. 10⁶ Pa 10⁷ Pa Bothsides ≦5 μm 20 60° C. 10⁶ Pa 10⁵ Pa None — 21 60° C. 10⁶ Pa 10⁶ Pa None— 22 60° C. 10⁶ Pa 10⁷ Pa None — 23 60° C. 10 ⁶ Pa 10⁵ Pa None — 24 60°C. 10⁶ Pa 10⁶ Pa None — 25 60° C. 10⁶ Pa 10⁷ Pa one side ≦5 μm 26 60° C.10⁶ Pa 10⁵ Pa Both sides ≦5 μm 27 60° C. 10⁶ Pa 10⁶ Pa one side ≦5 μm 2860° C. 10⁶ Pa 10⁶ Pa one side ≦5 μm

[0062] In TABLES 1 and 2, symbol 1 in the columns of materials ofpreviously fired substrate and unfired ceramic greensheet designates alow-temperature firable ceramic comprising a blend of 60 weight % glassof CaO—SiO₂—Al₂O₃—B₂O₃ system and 40 weight % alumina. Further, symbol2 designates a previously fired substrate made from the ceramicdesignated by symbol 1 on which a thick film resistor of rutheniumoxide (RuO₂) system is formed, the thick film resistor being trimmed foradjustment of a resistance value. Symbol 3 designates a low-temperaturefirable ceramic comprising a blend of glass of PbO—SiO₂—B₂O₃ system andalumina. “One side” in “structure” in the column of “cavity” designatesthe multilayer ceramic substrate having one side formed with a cavity asshown in FIG. 1C. “Both sides” designates the multilayer ceramicsubstrate having both sides formed with respective cavities as shown inFIG. 4.

[0063] A visual inspection was carried out for all the multilayerceramic substrates fired under restriction designated by sample Nos. 1to 28 in TABLES 1 and 2. As a result, no peeling occurred between thepreviously fired substrate and a fired layer of the unfired ceramicgreensheet, whereupon a degree of sintering was good.

[0064] Each of sample Nos. 1 to 16 designates a multilayer ceramicsubstrate having one side formed with a cavity fabricated in the methodof the first embodiment. The obtained multilayer ceramic substratehaving one side formed with a cavity had a good dimensional accuracy ineach of these samples since an amount of warp in the cavity bottom isequal to or smaller than 5 μm.

[0065] Sample No. 10 is a multilayer ceramic substrate fired with nopressure applied under restriction and having one side with a cavity. Inthis case, too, the restricting greensheet was bonded to thelow-temperature firable ceramic greensheet by thermo compression bondingin the step of stacking the restricting greensheet, whereby the obtainedmultilayer ceramic substrate had substantially the same quality as thatobtained by the firing under pressure.

[0066] Each of sample Nos. 17 to 19 designates a multilayer ceramicsubstrate having both sides formed with respective cavities fabricatedin the method of the second embodiment. The obtained multilayer ceramicsubstrate having both sides formed with respective cavities also had agood dimensional accuracy in each of these samples since an amount ofwarp in the cavity bottom is equal to or smaller than 5 μm.

[0067] In each of sample Nos. 1 to 19, both previously fired substrateand unfired ceramic greensheet were made from the low-temperaturefirable ceramic designated by symbol 1. On the other hand, in each ofsample Nos. 20 to 26, the previously fired substrate was made from aceramic material differing from the low-temperature firable ceramicdesignated by symbol 1. Thus, the previously fired substrate was madefrom a ceramic material differing from the material of the unfiredceramic greensheet. For example, in sample No. 20, the previously firedsubstrate was made from alumina as an insulating ceramic. In sample No.21, the previously fired substrate was made from a magnetic ceramic offerrite system. In each of sample Nos. 22, 25 and 26, the previouslyfired substrate was made from a dielectric ceramic of relaxor system.Further, in sample No. 23, a thick film resistor of ruthenium oxide(RuO₂) system was formed on the surface of the previously firedsubstrate made from the ceramic of 1, and the thick film resistor wastrimmed. The unfired ceramic greensheet was made from a low-temperaturefirable ceramic comprising a blend of glass of PbO—SiO₂—B₂O₃ system andalumina. In sample No. 24, the previously fired substrate was made froma dielectric ceramic of barium titanate. In sample No. 27, thepreviously fired substrate was made from a low-temperature firableceramic of 1, and the unfired ceramic greensheet was made from alow-temperature firable ceramic of 3. Further, in sample No. 28, bothpreviously fired substrate and unfired ceramic greensheet were made fromthe low-temperature firable ceramic of 3.

[0068] In each of sample Nos. 20 to 27, the material of the previouslyfired substrate differed from that of the unfired ceramic greensheet.

[0069] A visual inspection was also carried out for all the multilayerceramic substrates of sample Nos. 20 to 27. As a result, no peelingoccurred between the previously fired substrate and a fired layer of theunfired ceramic greensheet, whereupon a degree of sintering was good.Further, the obtained multilayer ceramic substrate having one or bothsides formed with respective cavities also had a good dimensionalaccuracy in each of these samples since an amount of warp in the cavitybottom is equal to or smaller than 5 μm.

[0070] Regarding sample No. 28, both previously fired substrate andunfired ceramic greensheet were made of the low-temperature firableceramic of 3. In this sample, too, the obtained multilayer ceramicsubstrate having one side formed with a cavity had a good dimensionalaccuracy since an amount of warp in the cavity bottom is equal to orsmaller than 5 μm.

[0071] On the other hand, TABLE 3 shows experimental results in the casewhere a multilayer ceramic substrate having one side formed with acavity was fired in a conventional method in which no restrictinggreensheet was used. TABLE 3 Comparative Cases Previously Unfired firedsubstrate ceramic greensheet Sample Thickness Thickness No. Materialafter firing Material before firing 29 (1) 0.4 mm (1) 0.5 mm 30 ↑ 0.4mm ↑ 0.5 mm Restricting Conditions for thermo Sample greensheetcompression bonding Pressure applied No. Material Temperature Pressureduring firing 29 None 100° C. 10⁶ Pa 0 30 None 120° C. 10⁷ Pa 0 CavitySample Amount of No. Structure warp in bottom Degree of sintering 29 oneside — Peeling occurred 30 one side — Peeling occurred

[0072] Regarding sample Nos. 29 and 30, the previously fired substrateand the unfired ceramic greensheet were bonded together bythermo-compression bonding. The stack was fired in the conventionalmethod in which no restricting greensheet was used. As the result of thevisual inspection, peeling occurred between the previously firedsubstrate and a fired layer of the unfired ceramic greensheet, whereupona degree of sintering was low. From the experimental results, it wasconfirmed that there is a high possibility that the conventional firingmethod using no restricting greensheet results in a low degree ofsintering.

[0073] The foregoing description and drawings are merely illustrative ofthe principles of the present invention and are not to be construed in alimiting sense. Various changes and modifications will become apparentto those of ordinary skill in the art. All such changes andmodifications are seen to fall within the scope of the invention asdefined by the appended claims.

I claim:
 1. A method of fabricating a multilayer ceramic substratecomprising: stacking one or a plurality of unfired ceramic greensheetson one or both sides of a previously fired ceramic substrate, therebyforming a stack, each unfired ceramic greensheet having a firingtemperature substantially equal to or lower than a firing temperature ofthe previously fired ceramic substrate; stacking a restrictinggreensheet on the unfired ceramic greensheet composing an outermostlayer of the stack, the restricting greensheet having a higher firingtemperature than each unfired ceramic greensheet; firing the stack atthe firing temperature of the unfired ceramic green sheets with orwithout pressure applied via the restricting greensheet while the stackis under restriction by the restricting greensheet, thereby integratingthe stack; and eliminating remainders of the restricting greensheetafter the firing step.
 2. The method according to claim 1, wherein thepreviously fired substrate and the unfired ceramic greensheets aretemporarily tacked together in the step where the unfired ceramicgreensheets are stacked on the previously fired substrate.
 3. The methodaccording to claim 1, wherein the restricting greensheet and the unfiredceramic greensheets are temporarily tacked together in the step wherethe restricting greensheet is stacked on the unfired ceramicgreensheets.
 4. The method according to claim 1, wherein the previouslyfired substrate and the unfired ceramic greensheets are made from a sameceramic material.
 5. The method according to claim 1, wherein thepreviously fired substrate and the unfired ceramic greensheets are madefrom different ceramic materials from each other, and the firingtemperature of each unfired ceramic greensheet is equal to or lower thanthe firing temperature of the previously fired substrate.
 6. The methodaccording to claim 1, wherein each unfired ceramic greensheet made froma low-temperature firable ceramic material which is fired at atemperature equal to or lower than 1000° C.
 7. The method according toclaim 1, wherein the previously fired substrate and/or each unfiredceramic greensheet are made from a ceramic having any one of insulationcharacteristic, dielectric characteristic, magnetism, piezoelectric, andresistivity.
 8. A method of fabricating a multilayer ceramic substratehaving a cavity using the method of fabricating a multilayer ceramicsubstrate according to claim 1, wherein the previously fired substrateis placed on a bottom of a portion where the cavity is to be formed, andan opening for forming the cavity is formed in the unfired ceramicgreensheet which is stacked on the previously fired substrate, before orafter stacking.
 9. A method of fabricating a multilayer ceramicsubstrate having a stepped cavity using the method of fabricating amultilayer ceramic substrate according to claim 8, wherein every time apredetermined number of the unfired ceramic greensheets corresponding toa number of layers of one step of the stepped cavity is stacked on thepreviously fired substrate, the restricting greensheet are stacked onthe unfired ceramic greensheet into the stack, and the stack is firedwhile being under restriction by the restricting greensheet, therebyfabricating a new fired substrate with one step of cavity, andthereafter, another predetermined number of the unfired ceramicgreensheets corresponding to a number of layers of a subsequent step ofthe stepped cavity and the restricting greensheet are stacked on saidnew fired substrate into a stack, and the stack is fired while beingunder restriction by the restricting greensheet, after remainders of therestricting greensheet for fabrication of said new fired substrate hasbeen eliminated, repeatedly so that the multilayer ceramic substratehaving the stepped cavity is fabricated.
 10. The method according toclaim 1, wherein a conductor pattern co-firable with the unfired ceramicgreensheet is printed on said unfired ceramic greensheet and thereafter,said unfired ceramic greensheet is stacked on one or both sides of thepreviously fired ceramic substrate.
 11. The method according to claim 1,wherein the multilayer ceramic substrate has a surface and a back eachone of which is dissimilar to the other.
 12. The method according toclaim 1, wherein a thick film resistor is formed on the previously firedsubstrate and the thick film resistor is trimmed for adjustment of aresistance value, and thereafter the unfired ceramic greensheet isstacked on the previously fired substrate.